1. Field of the Invention
The present invention generally relates to a display device and more particularly relates to a non-emissive display device that conducts a display operation using the light emitted from a backlight unit.
2. Description of the Related Art
Examples of non-emissive display devices include liquid crystal displays (LCDs), electrochromic displays, and electrophoretic displays. Among other things, LCDs are currently used extensively in personal computers, cell phones and many other electronic devices.
An LCD is designed to display images, characters and so on by changing the optical properties of its liquid crystal layer at the openings of its picture element electrodes, which are regularly arranged in a matrix pattern, with drive voltages applied to those electrodes. Also, in the LCD, each picture element includes a thin-film transistor (TFT) as a switching element in order to control the respective picture elements independently of each other.
However, if each picture element includes a transistor, then the area of each picture element decreases and the brightness drops.
Furthermore, considering their electrical performance, manufacturing techniques and other constraints, it is difficult to reduce the sizes of switching elements and interconnects to less than certain limits. For example, an etching precision achieved by a photolithographic process is usually about 1 μm to about 10 μm. Accordingly, as the definition of an LCD has been further increased and as the size thereof has been further decreased, the picture element pitch becomes smaller and smaller. As a result, the aperture ratio further decreases and the brightness further drops.
To overcome this low-brightness problem, according to a proposed method, a collecting element may be provided for each of the huge number of picture elements of an LCD so that the light emitted from a backlight unit is collected on each of those picture elements.
For example, Japanese Laid-Open Publication No. 2-12224 discloses a transmissive color LCD that uses microlenses. In the LCD of that type, a number of microlenses are arranged as densely as possible on a two-dimensional plane, thereby realizing a bright display as shown in FIG. 15B. Since the microlenses are arranged in such a densest possible pattern, R, G and B color filters (and their associated picture elements) are arranged in a delta pattern such that every set of RGB color filters on a row is shifted from its associated set of RGB color filters on the previous row by one and a half pitches as shown in FIG. 15A. That is to say, the ratio of the picture element pitch in the x direction (i.e., row direction) to the picture element pitch in the y direction (i.e., column direction) is 2: √{square root over ( )}3.
However, to realize the microlens arrangement disclosed in Japanese Laid-Open Publication No. 2-12224, the picture elements of an LCD need to be arranged in the delta pattern at the predetermined pitch.
An LCD with such a delta picture element arrangement achieves the display of a natural video, and therefore, can be used effectively in TV sets, camera finders and so on. However, to present characters, figures and other objects including a lot of lines on personal computers, cell phones and so on, the LCD preferably adopts not so much the delta arrangement as a striped arrangement. In the striped arrangement, normally three rectangular R, G and B picture elements make up one substantially square pixel as shown in FIG. 16. The microlens arrangement of Japanese Laid-Open Publication No. 2-12224 cannot be applied to any LCD with this striped arrangement.
There is an increasing demand for further improvement of optical efficiency in various other non-emissive display devices as well, not just the LCDs described above.